The invention relates to a method of determining the amount of starch used in surface-sizing a cellulose product.
Further, the invention relates to an arrangement for determining the amount of starch used in coating a cellulose product.
In manufacturing cellulose products, properties of a product can be influenced by using what is called surface sizing. In surface sizing, the surface of a cellulose product is closed by adding interfibre bonds with water-soluble polymers, such as starch, to improve strength properties, facilitate further treatment and prevent dust formation, for example. In surface sizing, starch is typically applied to the surface of the cellulose product in very small amounts, for instance 0.5 to 2 g/m2 either on one side or both sides of the paper. Surface sizing is typically used for, for instance, fine papers, raw papers to be coated and cardboards.
When determining properties and compositions of materials, one method generally in use is spectroscopy, which is based on analyzing radiation. In spectroscopy, radiation which is either radiated, absorbed or reflected by the object to be examined may be used according to the purpose of use. For different purposes, different wavelength ranges may be utilized, such as infrared radiation and ultraviolet radiation, and sectors of spectroscopy are correspondingly called infrared spectroscopy, i.e. IR spectroscopy, and ultra-violet spectroscopy, i.e. UV spectroscopy. Typically, many properties of cellulose products, such as the amount of actual coating, can be measured particularly by means of IR spectroscopy, as disclosed in patent publication FI 115 412, for example.
IR spectroscopy may be utilized when determining the composition of a material, for instance by measuring absorption of radiation, i.e. radiation absorbed by the material, at several wavelengths of infrared radiation. The absorbance of infrared radiation is proportional to the concentration of the absorbing substance in the sample, and the absorption peaks caused by the resonance frequencies of each particular chemical substance are located at wavelengths characteristic of the substance. Thus, the use of appropriate wavelengths enables determination of the composition and, for example, moisture of a cellulose product.
A problem in determining the amount of starch by utilizing spectroscopy is, however, that the absorption peak of starch is located substantially in the same wavelength range as the absorption peak of cellulose, due to which determining the amount of starch with a conventional application method of spectroscopy by direct measurement of the absorption value is not successful. One example of utilizing spectroscopy in determining the amount of starch-based size used for sizing layers of corrugated board is, however, disclosed in patent publication U.S. Pat. No. 5,663,565. When layers of corrugated board are sized, starch size is spread only at the ends of the folds on the contact surfaces to be sized, not evenly over the whole product surface. In the method of this publication, absorption values are measured in the absorption wavelength range of the water and/or starch contained by the size, and the signal part oscillating at the frequency of the folding is separated from the measurement signal, while the variations relating to something else than the amount of size are filtered. This method is not, however, applicable to measuring starch in surface sizing, where the intention is to spread starch evenly over the surface of a cellulose product. Further, the measuring inaccuracy of the method is great due to the number of factors affecting the measuring result.
Thus, in practice, measuring the amount of starch sufficiently accurately is almost impossible with the present measuring technology. Typically, the only method available in addition to the method of the abovementioned US publication is dry weight difference measurement, which determines the difference between the basis weight and the weight of water before adding surface size and after having added surface size, the amount of starch being determined on the basis of the difference between these two, i.e. on the basis of the dry weight difference. However, this method is problematic as well, because the error of measurement may, even with this method, easily rise to dozens of per cents of the amount of starch, which is typically very small as compared with the amount of cellulose. Furthermore, this method requires basis weight measurement, which utilizes ionizing radiation having possible adverse effects known as such, to be carried out both before and after the surface sizing.